In integrated circuits, it is often desirable to monitor the current being driven through a particular device. For example, an output transistor may require current limiting to protect the integrated circuit in the event that a fault develops in the external circuit. As another example, some implementations of switch-mode power controllers require a measure of inductor current as an input to the control circuitry. These applications, as well as many others, require that the current through a particular device of the integrated circuit be monitored. Often the current through a FET transistor must be monitored to provide the desired functionality.
Various prior art approaches to this problem exist. FIGS. 1 and 2 are schematics of two prior art current sensing circuits. In FIG. 1, transistor M1 is an NMOS power transistor which is connected between the low-potential side of a load at terminal OUT and ground. This configuration is usually called a low-side-drive (hereinafter LSD) configuration. The NMOS transistor M1, when enabled by a voltage at the circuit input terminal VIN, will pull output terminal OUT to a low voltage. A small resistor R1 is inserted between M1's source and ground. The voltage V1 developed across resistor R1 is simply: EQU V1=IL.multidot.R1. [1]
Thus the resistor is used to generate a voltage proportional to the current flowing in the output transistor M1. Stated another way, the resistor R1 is a current sensor. The circuit of FIG. 1 can be coupled to a comparator to create a current limiting circuit. The disadvantages of using the current sensing scheme of FIG. 1 in an integrated circuit are well known. Building precise resistors is very difficult in most process technologies used in fabricating semiconductor devices. Further, the resistor should preferably be of a fairly low value so as to limit unnecessary power dissipation. Building low value resistors is also difficult or impractical, particularly if matching to a higher-value resistor is also required.
Current sense resistors, such as are used in the circuit of FIG. 1, are not desirable components. They introduce Ohmic losses which would otherwise have been avoided, and they are often difficult to construct. Even if metal resistors are used, a substantial number of squares of metal may be required to form the current sense element. The ideal current limit circuit would require no current sense resistor.
For FET transistors, it is possible to use the inherent on-resistance of the transistor as a sense element. To understand why this is so, consider the Shichman-Hodges equation for a MOS transistor in the triode (or linear) region: EQU Id=k.multidot.(Vgs-Vt-Vds/2).multidot.Vds (Vds&lt;Vgs-Vt) [2]
where k=k'.multidot.(W/L).
For most applications where a power MOS transistor is used as a switch, the voltage across the transistor will be quite small relative to the voltage (Vgs-Vt). This not only ensures operation in the triode region, but it also allows the term Vds/2 to be neglected in equation [2], simplifying the square-law equation to a linear equation of the form: EQU Id.apprxeq.k.multidot.(Vgs-Vt).multidot.Vds (Vds&lt;&lt;Vgs-Vt) [3]
The on-resistance of the transistor, Rds.sub.on, is merely the ratio Vds/Id, which leads to the approximate formula for Rds.sub.on : EQU Rds.sub.on .apprxeq.1/(k.multidot.(Vgs-Vt)) [4]
Obviously the variabilities inherent in the on-resistance Rds.sub.on of equation [4] are formidable. Both k and Vt vary with process and temperature, and Vgs is dependent upon the gate drive voltage supplied to the transistor. As a result, Rds.sub.on is not even approximately constant. Various schemes have been used to attempt to compensate for these variations.
A prior art current sensing circuit which attempts to solve the problems of the circuit of FIG. 1 by using the transistor on-resistance instead of a sensing resistor is depicted in FIG. 2. In the circuit of FIG. 2, the process and temperature dependent variabilities inherent in Rds.sub.on are handled by matching the Rds.sub.on of the power device to the Rds.sub.on of a reference device, which is preferably buried within the power device structure. Thermal matching is maximized by using similar devices which are in close physical proximity and therefore subject to the same process and temperature conditions.
In FIG. 2, NMOS output driving transistor M1 is again configured as a low side drive circuit. Any FET transistor can be used. When a positive voltage is applied at circuit input terminal VIN, the output terminal OUT will be pulled down to a low voltage. An operational amplifier (hereinafter op-amp) 2 is coupled to the voltage V1 at the output terminal OUT. This op-amp is set up in a unity gain configuration from node V1 to node V2. Transistor M3 is coupled to a current output IA. Transistor M2 is an FET transistor which matches M1, in other words it has similar device parameters and characteristics and it is assumed M2 has the same gate-to-source voltage Vgs as M1. Transistor M2 is used to sense the current flowing in the output transistor M1.
Because the voltages at nodes V1 and V2 are proportional to the currents flowing through the transistors M1 and M2, the voltages may be stated as: EQU V1=IL.multidot.Rds.sub.M1 EQU V2=IA.multidot.Rds.sub.M2 [ 5]
where the on resistance Rdson for a transistor is proportional to the length to width ratio (L/W). It is clear also that EQU V1=V2 [6]
because of the operation of op-amp 2. Since it is also true that M2 and M1 are matched transistors, it can be stated that EQU IA=IL.multidot.[(Rds.sub.M1)/(Rds.sub.M2)]=IL.multidot.[(L/W).sub.M1 /(L/W).sub.M2 ] [7]
The disadvantages with the scheme of FIG. 2 are that the circuit requires the use of an op amp circuit. These circuits are quite complex and involve many components. The operational amplifier has a relatively low bandwidth, consumes power, and the area required to implement it is high. The operational amplifier is therefore not ideal for integration with other circuitry.
The current sensing schemes of the prior art have limitations that make them unattractive for integrated circuit applications. Thus there is a need for an improved current sensing scheme for use in power integrated circuits and systems.